Collagen is a fibrous protein which is present in rind, cartilage, bones and hide. Currently, 28 different types are known. Under physiological conditions, collagen is insoluble, but can be hydrolyzed by heat, bases or weak acids. If a hydrolyzed collagen is purified, concentrated, sterilized, dried and milled, this produces gelatin, the special property of which, being able to bind considerable quantities of water, makes it widely used in a number of foods and medicaments.
In contrast thereto, collagen hydrolyzate is enzymatically hydrolyzed collagen which is also soluble in water, but, unlike gelatin, is no longer able to bind considerable amounts of water. However, it is readily dispersible and emulsion-stabilizing, which is why it is used in the cosmetics industry just as in the textile and food sectors, for example in the meat product, confectionery and drinks industries. In pharmacy, collagen is used as tableting compound, coating agent and filler.
The later use of the collagen plays a critical role even in the production of the collagen mass. This applies particularly in the use of collagen for sausage casings. The raw material for producing collagen skins (=hide fiber skins), which, depending on the casing thickness can also be suitable for consumption, is obtained from animal hide, preferably cattle hide. From the inside of the unhaired and freshly salted or lightly limed hide, the hide split is separated off (=split off). The resultant “lower split” is first subjected to an alkaline digest at a pH of about 13, for example with aqueous potassium hydroxide or calcium hydroxide. In this process the hide matrix is loosened and becomes more flexible. By adding acid until a pH of below 3.5 is achieved, the digest is ended. The resultant “collagen rinds” are coarsely minced and the minced material can then be forced through a plurality of perforated disks arranged in series, wherein the diameter of the individual holes becomes smaller from disk to disk. In this manner, a collagen paste is obtained which is transferred to large kneaders.
In the production of collagen skins which are not to be stuffed until later with sausage emulsion or another extrudable food, the collagen mass is frequently further mixed with cellulose fibers which give the skins a higher strength. The fraction of cellulose fibers is expediently about 10 to 25% by weight, based on the weight of the collagen. The mass is then extruded through an annular die and the tubular casing formed is subsequently stabilized. In this process use can be made of a dry spinning process or a wet spinning process. In the dry spinning process, a collagen mass having a relatively high content of dry matter is used. As a result, the tube already has sufficient stability directly after extrusion. For this, generally, use is made of an extrusion mass having a fraction of collagen dry matter of about 8 to 15% by weight, in particular of about 9% by weight (per DE 23 14 767, whose United States equivalent is U.S. Pat. No. 4,081,562 and DE 23 36 561, whose United States equivalent is U.S. Pat. No. 4,001,442). Dry-extruded collagen casings in addition, may be shaped under the action of heat. Unlike in the wet spinning process, edible ring skins may also be produced in such a manner. In the wet spinning process, in contrast, the dry matter content in the collagen mass is less. It is generally about 3 to 6% by weight. The mass is extruded in this case into a coagulation bath which contains an aqueous ammonium sulfate or sodium chloride solution and in addition ammonia as coagulating medium (see B. A. Lang, G. Effenberger, Wursthüllen—Kunstdarni [Sausage casings—artificial skin], Deutscher Fachverlag, Frankfurt a. M., 3rd edition [2006] pp. 58-63).
For the production of edible collagen skins, i.e. collagen skins suitable to be consumed as well, conventionally acidified, aqueous collagen masses having a collagen dry matter content of 3 to 6% by weight and about 0.6 to 1.2% by weight of cellulose fibers are used (per U.S. Pat. No. 4,615,889 and EP-B 0 821 878). They must be constantly cooled during storage and transport, in order that the spread of microorganisms causing breakdown is prevented. The longer the time period of the storage and transport is, the greater the additional risk that the acid present in the mass attacks the collagen and chemically destroys it. A further disadvantage is that the mass includes about 95% by weight water, which produces high transport costs based on the value of the collagen fraction. Since a large part of the collagen traded on the world market originates from South America, the transport costs are a significant economic factor. In addition, storage is complex. Refrigerated rooms and refrigerated containers are required.
Alternatively, the hide can be transported dried or salted. The salt, however, must be washed out again before further processing.
In addition, a process is known in which a food, in particular sausage emulsion, is extruded and at the same time a layer of a collagen gel is extruded around the food (per EP 0 618 771 B1). The aqueous collagen gel preferably contains about 4 to 10% by weight of collagen, in addition it can contain further small amounts (0.1 to 2.0% by weight) of cellulose. It preferably has a pH of 1 to 4. In this pH range the collagen gel takes up a particularly large amount of water (90 to 95% by weight). The collagen gel is subsequently crosslinked chemically, for example by a crosslinking agent mixed with the collagen gel, such as glutaraldehyde, glyoxal, liquid smoke, sugar or a mineral tanning agent. Coagulation can also be performed by treatment with a salt solution which removes water from the collagen gel, or by increasing the pH until it is in the isoelectric range of the collagen.
A very similar coextrusion process is disclosed in EP 1 130 978 B1. In this process, an activated-carbon-pretreated, substantially colorless liquid smoke having a pH of 5 to 7 is used for hardening the collagen. In the coextrusion process according to WO 2006/051278, an aqueous salt solution which additionally contains tartaric acid is used, instead of liquid smoke or another crosslinker, for hardening the casing produced from the collagen gel.
In the coextrusion processes, a collagen gel produced by processes known in principle is used. The collagen fibers are in this case comminuted until they form a very fine suspension (WO 2006/051278). Collagen concentrates in conjunction with coextrusion processes are not disclosed.